Until now, Liam Neeson has been master of his own domain, as a veteran thespian-turned-action star, a man who can invest his characters with authenticity in between the shootouts and explosions.

But now a new guy's on the block, and a two-time Oscar-winner to boot: Sean Penn.

In truth, despite Penn's acting chops and a more youthful physique, Neeson's position isn't under serious threat. One couldn't accuse The Gunman of firing blanks, but it does plod by the numbers.

Ironically, the same director helms The Gunman as Neeson's career-changing thriller Taken, the Frenchman Pierre Morel. And there are shared, overly familiar ingredients: the former assassin who won't be allowed to forget his past, the loved one in peril, the huge body count.

One difference is The Gunman's urge to be politically aware. Penn, who has a co-writer's credit and is well-known for his humanitarian engagement in Haiti, is almost certainly responsible for his film's attempt to underpin the violence with serious themes, regarding political and corporate skulduggery in the Third World. But ultimately these themes are no more than window dressing to the action, to the extent that their presence becomes almost trite.

It opens in 2006 in the Democratic Republic of Congo, as the country is engulfed in a civil war with the wealth of natural resources as the prize. Ex-special forces soldier-turned-military contractor Jim Terrier (Penn) is one of a team that includes Felix (Javier Bardem) and Cox (Mark Rylance), ostensibly offering security to an NGO but with a nefarious subplot to their activities. These include the assassination of a political leader, after which Terrier flees the country, leaving his girlfriend Annie (Jasmine Trinca) to the protection of his pal Felix.

Eight years later Terrier is back in the Congo, this time working legitimately for an NGO in an apparent desire to make amends. But when an attempt is made on his life, he's drawn back towards his past and those people he thought he'd left behind.

Hereafter, the film enters jet-setting chase mode, from Africa to London, Barcelona and Gibraltar. Morel has orchestrated some decent action sequences, involving his outnumbered hero surviving one tight spot after another. But there isn't one that creates a genuine adrenalin rush, while around the action the story is standard and the dialogue veers between cliché and unintentional hysteria.

Terrier's brain damage is a clunky plot device that is probably a throwback to the film's source, the French crime thriller The Prone Gunman, whose hero loses his ability to speak. With the book's noirish overtones and mystery element, it sounds as though a more faithful adaptation would have been wise.

At the same time, this brings to mind another recent screen thriller, The American, featuring George Clooney's assassin-turned-target, which had a minimalist and existential bent much more satisfying than the melodrama and hyper-violence on offer here.

A rich supporting cast, including Ray Winstone and Idris Elba, is woefully underused. The exception is Rylance, riding the wave of TV's Wolf Hall with another distinctive villain, this one with a straw hat and paunch that make him appear to be a middle-aged holiday-maker, but whose Joker's grin sends shivers down one's spine.

As for the star, he cuts an odd presence. Penn is perfectly able in everything he does. And he's in good shape. Yet that's partly the problem - while Winstone and others show their characters gone to seed, there are so many scenes involving Penn's beefed-up naked body that the film begins to smack of vanity project. I really expected more.